The present invention relates to a mounting scheme for a gradient magnet assembly, as used in Nuclear Magnetic Resonance (NMR) devices such as Magnetic Resonance Imaging (MRI) apparatus. In particular, but not restricted thereto, the present invention relates to a method of mounting annular coils to a cylindrical structure.
In typical NMR apparatus, first and second magnetic coil systems are arranged coaxially with respect to a longitudinal axis defined by a field direction of a homogenous magnetic field generated by the first magnetic coil system. The second magnet coil system comprises a gradient magnet assembly, which is operable to effect pre-defined magnetic fields about a patient/sample: the effect of the patient/sample on the magnetic field is monitored whereby to provide a magnetic resonant image. In MRI systems such an image can comprise a section through a patient. In non-medical NMR systems, such an image can indicate the presence of certain elements. The first magnetic system may further comprise a shielding set of magnets.
MRI is becoming an increasingly accepted diagnostic procedure in medicine. FIGS. 1 and 2 schematically show an MRI apparatus in perspective and part cross-sectional views. The apparatus has an outer skin 10 which comprises an external wall of a cryogenic insulating environment (cryostat) for the super-conducting magnet coils which produce a primary magnetic field, which operate at extremely low temperatures, such as four Kelvin. Within the cryostat are placed primary magnetic coils 16. The length L of the wall of the cryostat is typically around 1.5 m for whole body scanners. The gradient magnet assembly 14 is fixed to the inside wall of the cryostat. A gradient magnet assembly will typically comprise gradient magnet coils that are arranged orthogonally and may comprise three sets: one set for each of the orthogonal directions or six sets: two for each of the orthogonal directions. Other configurations of gradient magnets are possible. In operation the gradient coils are supplied with large currents, which are switched rapidly on and off to provide position encoding data necessary for the imaging process. In the presence of the strong primary magnetic field the resulting forces on the gradient coils cause substantial noise and vibration.
Conventionally, rigid mounts 18 in the form of wedges have been employed to separate and retain physically the gradient assembly from the inside of the cryostat wall. Such wedges do not isolate the cryostat from the mechanical vibrations associated with the gradient assembly. This can lead to vibrations being set up within the cryostat structure which can lead to a deterioration of the performance of the imaging system as a result of the effect of eddy currents induced when the conducting surfaces within the cryostat are moved relative to the primary magnetic field. It can also lead to an increase in the level of acoustic noise produced as the surfaces of the cryostat are vibrated. The rigid wedges may alternatively be replaced by resilient rubber mounts. Nevertheless, the need for the gradient assembly to be held firmly within the system means that no improvement has been found to be substantial.
Alternatively, the gradient assembly is supported with respect to a floor, separately from the support feet of the cryostat. This has the effect of lengthening the system: long systems tend not to be patient-friendly since confined diagnostic spaces can be distressing to some patients. Still other systems provide stiffness in one support direction and a much reduced stiffness in another direction.
Thus noise transmission paths from the gradient assembly and the mechanical mounts of the gradient assembly can be a significant problem.
The present invention seeks to provide an improved mounting system for magnets and, in particular, gradient magnet assembly for MRI apparatus.
In accordance with a first aspect of the invention there is provided a magnetic resonance imaging apparatus comprising a primary superconducting magnet, a cryostat and a gradient magnet assembly; wherein the primary superconducting magnet is contained within the cryostat and, wherein the gradient magnet assembly is spaced from and indirectly attached to the cryostat by tensionable straps. In a preferred embodiment the gradient magnet assembly is supported by straps wrapped around the cryostat. Conveniently these straps comprise a braided material. Alternatively the material may comprise a band, wire or ropes. Braided material or band material (whether woven or otherwise) may be manufactured from a steel such as a stainless steel, carbon fibre, polyamide, Kevlar (RTM) or other suitable material.
The MRI apparatus can comprise a tubular magnet, employing one or more solenoid coils. These solenoid coils may be arranged in a nested fashion, or extend along a longitudinal axis.
The straps may be configured whereby one or more surrounds a section of the cylindrical bore. The straps may be configured such that they extend about the inside of the bore, being connected at either end with the bore.
Thus, the invention can provide a mounting system for a tubular magnet within a bore, wherein the gradient magnet assembly is supported adjacent a wall of the bore by an arrangement of straps, the straps being configured and tensioned whereby to prevent direct contact of the gradient magnet assembly with a wall of the bore.
In an MRI magnet apparatus, a gradient magnet can thus be substantially dynamically de-coupled from a cryostat, without floor mounting.